In vitro activity: As Zibotentan specifically inhibits ETA-mediated antiapoptotic effects, but not ETB-mediated proapoptotic effects in human and rat smooth muscle cells, Zibotentan binds to endothelin A receptor (ETA) with high affinity with Ki of 13 nM, and has no affinity for endothelin B receptor (ETB) with IC50 of>10 μM. Zibotentan treatment at 1 μM inhibits ET-1 induced mitogenic activity in ovarian carcinoma cell lines HEY and OVCA 433 secreting ET-1 and expressing ETA and ETB mRNA. ZD4054 (1 μM) inhibits ET-1 induced EGFR transactivation in HEY and OVCA 433 cells. Zibotentan (1 μM) reverts ET-1 mediated epithelial-mesenchymal transition (EMT), by enhancing E-cadherin expression and promoter activity, and inhibiting vascular endothelial growth factor (VEGF) secretion and invasiveness in HEY and OVCA 433 cells. Zibotentan also potently inhibits the basal and ET-1 induced cell proliferation in SKOV-3 and A-2780 cells, associated with the inhibition of AKT and p42/44MAPK phosphorylation, and with increased apoptosis through the inhibition of bcl-2 and activation of caspase-3 and poly(ADP-ribose) polymerase proteins.

Kinase Assay: The inhibition by Zibotentan (varying concentrations) of 125iodine-ET-1 binding to cloned human ETA is assessed using standard radioligand-binding techniques. Human recombinant ETA is expressed in mouse erythroleukaemic cells, and cell membranes prepared for competitive binding studies using 125iodine-ET-1 as the radioligand. Incubations are carried out in triplicate in the presence of Zibotentan, 100 pM to 100 μM in half-log increments, and inhibition of ET-1 binding is expressed as the geometric mean pIC50 value (concentration to inhibit 50% of binding) with a 95% confidence interval (CI). The affinity of Zibotentan for cloned human ETA is also assessed using the equation of Cheng and Prusoff to determine the equilibrium dissociation constant (Ki) in a further receptor-binding screen utilizing a greater number of concentration-response curves determined in three separate studies.

Cell Assay: Cells (HEY and OVCA 433) are serum starved by incubation for 24 hours in serum-free DMEM before exposed to Zibotentan for 48 hours. After the treatment, cells are lysed and the supernatant is recovered and assayed for histone-associated DNA fragments, at 405 nm by the use of a microplate reader. For detection of early apoptotic events, floating and adherent cells are collected. Cells are double stained with FITC-conjugated Annexin V and propidium iodide using the Vybrant Apoptosis Kit and are immediately analyzed by cytofluorometric analysis.

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